Michigan City police and firefighters dig with shovels to rescue Nathan Woessner, of Sterling, Ill., who was trapped for over three hours under about 11 feet of sand at Mount Baldy dune near Michigan City, Ind. The grandfather of a 6-year-old boy says he is responsive and able to move his arms and legs.

CHICAGO — The National Park Service is investigating what caused a hole in an Indiana dune that swallowed a 6-year-old boy, trapping him for more than three hours beneath 11 feet of sand before rescuers could reach him.

Some geologists theorize that a long-buried tree trunk decomposed and created the void — and possibly an air pocket that kept the boy alive — in the dune known as Mt. Baldy, and hope to use ground-sensing equipment to peer beneath the surface, said Bruce Rowe, a ranger at the Indiana Dunes National Lakeshore.

Nathan Woessner of Sterling, Ill., was unresponsive when he was found last Friday, but began breathing en route to a waiting ambulance. He remained in critical but stable condition Wednesday at Comer Children’s Hospital in Chicago, where doctors said he is expected to make a full neurological recovery, though he may suffer lingering lung problems from inhaling sand.

“At this point, we still don’t know what caused the hole,” Rowe said, though the tree theory seems plausible because the dune moves about 4 to 10 feet a year and “it’s covered a lot of trees.”

“We’ve never seen evidence of a hole associated with it,” Rowe said.

Rescuers reached Nathan shortly after one of them inserted a probe into the sand in an area that looked like the outline of a tree trunk. A tree was not found, though “quite a bit of bark” was, leading to speculation that the sand many feet below the surface might have been wet enough to hold the shape of a long-decayed trunk, Rowe said.

“But there certainly is no conclusion that we can draw at this point,” he said.

Another potential explanation is that sand and debris from a higher elevation washed out underground and discharged at a lower elevation — a phenomenon known as “piping” — creating a subterranean cavity, said Sam Panno, senior geochemist at the Illinois Geological Survey. He said he has heard of people occasionally falling into such cavities, though many of the formations are stable or collapse on themselves.

He said cavities can form quickly, making it difficult to detect them and impractical to scan for them often with ground-penetrating radar or other technology.

Rowe said a team of geologists and others will meet today to determine the next steps in the investigation.

Mt. Baldy will be closed indefinitely, he said. About two-thirds of the dune already was roped off to try to keep visitors from trampling native dune grasses and other vegetation that had been planted to help keep the sand in place, Rowe said. The dune historically moved about 4 feet southward each year, but in recent years has moved 10-12 feet, he said.

The 123-foot-tall Mt. Baldy is the tallest of the dunes at the Indiana Dunes National Lakeshore, which runs for about 25 miles along the southern shore of Lake Michigan and attracts families, hikers and birdwatchers.

Nathan was climbing the dune with an 8-year-old friend and their fathers last Friday when the friend yelled that Nathan had disappeared. Rescuers used backhoes, shovels and their hands to reach the boy.

Doctors have said Nathan could be taken off a ventilator by the end of the week and released from the hospital within two weeks, but may need another month in a rehab facility.